RICHLAND, Wash. (AP) - A strongly worded new report from a congressional watchdog agency questions whether an often-delayed and increasingly expensive waste treatment plant at the nation's most contaminated nuclear site can be built and operated successfully.

Released Friday, the report by the Government Accountability Office criticizes the U.S. Department of Energy and those building the plant for their performance during construction of the facility at the Hanford nuclear reservation in south-central Washington.

"By just about any definition, DOE's (waste treatment plant) project at Hanford has not been a well-planned, well-managed, or well-executed major capital construction project," the GAO report said.

"Daunting technical challenges that will take significant effort and years to resolve, combined with a near tripling of project costs and a decade of schedule delays, raise troubling questions as to whether this project can be constructed and operated successfully."

Earlier this week, Washington state and federal officials said construction can begin to be ramped up again. That word came nearly seven months after a new cost estimate and construction schedule were delayed to address technical problems and safety concerns.

Design of the $12.3 billion plant is 85 percent complete while construction is more than 50 percent complete.

The plant has long been considered the cornerstone of cleanup at Hanford. However, the Energy Department, which manages cleanup at the site, halted construction on significant portions of the plant amid technical issues and concerns about its eventual safe operation.

Then-Gov. Chris Gregoire and Energy Secretary Steven Chu said in a statement Tuesday that the state and federal government have worked closely to resolve those issues in recent months and that, based on information gathered from leading scientific experts, the Energy Department is confident that construction can be ramped back up again.

The federal government created Hanford in the 1940s as part of the top-secret Manhattan Project to build the atomic bomb.

The plant will convert millions of gallons of highly radioactive waste into glasslike logs for permanent storage. That waste is currently stored in 177 aging, underground tanks, many of which have leaked into the groundwater, threatening the neighboring Columbia River.

The plant's design includes new technology that will need to operate for decades "with perfect reliability," the GAO report said, because high radiation levels won't allow access for maintenance and repair.

The Oregonian reports that the GAO recommended against resuming construction until key milestones are met, including verifying critical technologies.

Responding to the report, the Energy Department said it would allow limited construction, such as completing structural walls at the high-level treatment plant, while "taking into account the remaining open technical issues."

The accountability office said that approach "appears reasonable," the newspaper reported.

DOE also said it would work to improve contractor performance evaluations, and would review incentive payments made since January 2009.